Extended data Figure 7. EPR/ENDOR characterization, see Supplementary information.

a) Q-band ENDOR and HYSCORE spectra of the spectral region where an ¹⁴N hyperfine coupling should be observed. Experimental parameters are listed in the materials and methods section.

b) Full multifrequency (X-/Q-band) EPR dataset and corresponding field dependent Q-band ENDOR spectra. Experimental parameters are listed in the materials and methods section. The red dashed lines represent a simultaneous simulation of all datasets using the spin Hamiltonian formalism. Simulation parameters are listed in Supplementary information Table 1.

c) Inferred orientation (θ₁, θ₂) of the C_β protons relative to the phenoxyl radical ring plane as determined by the dihedral angle (ϕ) between the ring plane (C1) and C_α.

d) Candidates discussed for the MfR2 radical species, see Supplementary information.

All ENDOR measurements were repeated at a second microwave frequency (W-band) giving similar results. Pulse EPR and ENDOR measurements represent long data accumulations/averages. EPR: 300 averages (6 scans/50 shots). ENDOR: 600 averages (600 scans/1 shot).